1. Field of the Invention
The present invention relates to NOx sensors for nitrogen oxides in gas mixtures and more particularly to sensors directly exposed to automotive emissions to measure a total concentration of NOx. The invention is naturally applicable as apparatus for monitoring the NOx concentration of emissions from ordinary manufacturing plants and for environmental monitoring inside tunnels.
2. Description of the Prior Art
Inasmuch as oxides of nitrogen (NOx) in automotive emissions consist essentially of nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2), these two types of gases are subject to detection of the total NOx concentration in the emissions from automotive engines. A construction of a prior-art NOx sensor for detection of automotive emissions which is designed to detect the total NOx concentration is disclosed in FIG. 1 ("Thick Film ZrO.sub.2 NOx Sensor" as announced by NGK Insulators, Ltd. in February, 1996 at the Society of Automotive Engineers, Inc.).
It is considered that the following principle underlies the sensor structure in FIG. 1. Namely, partial pressure of oxygen in a first cell 12 (gas flow-in side) in zirconia substrates (having a solid solution of 3-8 mol % Yttrium) 1, 2, 3, 4 is completely diminished to zero or to a constant value by adjustng the voltage of the oxygen pumping electrodes 6a, 6b according to output of oxygen concentration sensors 9a, 9b of the concentration electromotive force type provided at a second cell 13, whereupon only NO.sub.2 of the emissions is reduced to NO. Subsequently, NO of the first cell 12 diffuses and flows into the second cell 13, wherein the NO is completely decomposed according to formula (1): EQU NO+2e.sup.- .fwdarw.N+O.sup.2- . . . (1)
Oxygen (ion) which is dissociated then is discharged at second electrodes 16a, 16b to outside the cell. An oxygen ion current obtained at this time is measured to detect the NOx concentration therein. It is to be noted that an electrode voltage appropriate as an oxygen ionization voltage is set up in the second electrodes. Numeral 10 indicates a gas inlet and numeral 11 indicates a gas diffusion aperture. A space 14 between the substrates 2, 4 is a space leading to the atmosphere.
It is basically possible to detect the total NOx concentration of the automotive emissions by using the NOx sensor having the above-mentioned construction in FIG. 1. Nevertheless, there are problems as described hereunder in consideration of the actual situation wherein the sensor thereof is put to use.
Consider first the detection output in accordance with the construction of FIG. 1. The amount of dissociated oxygen through decomposition of NO in the second cell depends on the NOx concentration in the emissions as well as the amount of NO which can flow into a detection cell. The NOx concentration as such in the emissions amounts from 10 ppm to 100 ppm at the most, whereas the issued NOx concentration to be detected in practical use is under 100 ppm in a large number of cases. It must be pointed out that though this method provides the a linear output with the NOx concentration, it is difficult to make an accurate measurement of NOx in the low concentration region below 100 ppm.
Further, the method of FIG. 1 causes an offset since the remainder of oxygen in the second cell directly adds to the sensor output, wherefore there is a restriction that the partial pressure of oxygen inside the second cell be subjected to the control of oxygen partial pressure with an accuracy corresponding to the measurement accuracy (of order of under 1 ppm).
Still further, unless the mode of NO gas diffusion into the second cell is in terms of limiting current because the sensor thereof is of the current detection type, changes in the electrode characteristics will directly affect signal current. That is, output fluctuation with the passage of time is considered to be substantial. In actuality, if the limiting current is to be obtained at the NOx concentration level in the emissions, the output level will be small to a great degree to lessen the plateau output displacement amount further, thus leading to additional reduction of the resolution in measuring gas concentration. Accordingly, the sensor of FIG. 1 is not suitable for practical applications in automotive emissions.
As described above, the problem of the sensors which can directly detect the total NOx concentration in the automotive emissions is the development of stable sensors that can detect, with high accuracy, the total NOx concentration under 100 ppm.